The present invention relates to a system and a method for estimating storage capacity of an exhaust gas aftertreatment device, and more particularly, to estimating an oxidant storage capacity of a Lean NOx Trap coupled downstream of an internal combustion engine.
Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream three-way catalyst. The LNT stores exhaust components such as, for example, NOx and oxygen, when the engine is operating at a lean air-fuel ratio, and releases and reduces (purges) them when the engine is operating at a rich or stoichiometric air-fuel ratio. Over time, the ability of the LNT to store exhaust components can decrease due to factors such as sulfur deposits (SOx) from the fuel. Therefore, when the LNT efficiency is sufficiently reduced, a SOx purge has to be performed. Typically, the catalyst is heated and engine air-fuel ratio is changed to rich for SOx release and reduction. Since SOx purges result in fuel economy penalties, it is desirable not to purge unnecessarily. Thus, in order to maintain adherence to emission standards and obtain fuel economy benefits of a lean burning engine, it is desirable to monitor the efficiency of the LNT.
One method of determining the efficiency of the LNT is by correlating it to the oxidant storage capability, as described in U.S. Pat. No. 5,713,199. The amount of stored oxidant is calculated from a quantity of fuel required to purge it from the LNT wherein the purge is performed during high load operating conditions.
The inventors herein have recognized a disadvantage with this approach. Namely, when estimates of this method are performed under high load operating conditions, i.e., at high space velocity, the amount of time that the reductant used to purge stored oxidants is present in the LNT is reduced. Therefore, not all of the oxidants are purged during high-speed engine operation, resulting in an inaccurate estimate of the total LNT storage capacity. In other words, the reductant does not have sufficient time to react with stored oxidants when the LNT purge is performed under high load conditions. This will cause inaccurate capacity estimates and reductant wastage.
In solving the above problem, a system and a method are provided for monitoring efficiency of an exhaust gas aftertreatment device by correlating it to the device""s capacity to store an exhaust component.
In carrying out the above solution, features and advantages of the present invention, a system and a method for monitoring an efficiency of an exhaust gas aftertreatment device disposed in an exhaust passage of a lean burn internal combustion engine, include providing an indication that the engine is operating at low space velocity; and in response to said indication, estimating an amount of an exhaust component stored in the device.
The present invention provides a number of advantages. For example, it allows for much more accurate estimate of oxidant storage capacity, since at idle or low toad operation the reductant injected into the device has more time to react with the stored oxidants than during high speed operation. Therefore, all of the oxidants stored will be released, and wastage of reductant will be eliminated. Thus, due to increased accuracy of oxidant storage estimation, the efficiency Of the LandJT can be determined with more precision and less reductant wastage. This in turn will prevent unnecessary purges allowing for more lean running time, and increase fuel economy. Also, improved emission control can be achieved due to improved accuracy.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.